The invention relates to a proximity detector whose object is to reveal the presence of an object at a certain distance from an element, as, for example, of an intruder in the proximity of a building entrance or of an obstacle in the trajectory of a moving body such as the tool of a press.
While not exclusively, it relates more particularly to the application of this detector to the vertical edges of power operated doors and especially to hoistway doors and/or doors of elevators and hoists. In this particular application, the object is obviously to detect the presence of a person or of an object that might be injured while the door is still at a certain distance, so that, as a result of the signal emitted by this detector, is is possible to block the door and thus to interrupt its closing or even effect its reopening if the level of this signal exceeds a certain threshold.
For this application to the reopening of elevator doors, there already exists various devices tending to limit if not to prevent damage. In particular:
strain detectors, which however detect the presence of the person or of the object only by running into it;
pneumatic feelers which, while very sensitive, are also very fragile;
photoelectric cells, whose area of sensitivity is limited to the sector scanned by the beam of light;
radars which, in addition to being very expensive, present the drawback of not being selective.
Proximity detectors are also known (U.S. Pat. No. 3,018,851) comprising a very low-frequency low-energy oscillator whose emission point is grounded so as to emit a signal of constant amplitude and frequency,
at least one receiving antenna, divided into several independent sections, mounted on the part of the element whose approach is being monitored but located outside the direct radiation field of the elements that surround this part and which are grounded but which maintain an unvarying distance from said part, and
a processing circuit for the voltage induced in each section of the receiving antenna by the waves reflected and radiated by the bodies that surround the antenna, so as to emit a final signal whose intensity will be modified by the presence of a foreign object in proximity of said antenna.
Detectors of this type operate by reading differences in the capacitive coupling to the ground of each antenna section and are well suited to solve the problem of detecting an obstacle to the closing of the doors. Unfortunately, in detectors of this type known to date, the processing of the signals emitted by the antenna sections operates by balancing a bridge circuit (U.S. Pat. Nos. 3,018,851 and 2,720,284), as a result of which the system is very unstable and requires complex adjustment. That is why one result that the present invention achieves is a very stable detector, which is simple to adjust, inexpensive and easily mounted on the element in whose proximity is to be detected the approach of any body and, for example, on specially designed or already existing doors.